The lifetime of an LED fixture is one of the huge selling points over traditional fixtures like incandescent and fluorescent. An LED lamp or, more familiarly, LED “light bulb” or simply “LED” can last a long time (on the order of tens of thousands of hours) when designed in the correct environment. The lifetime of an LED (and power supply driving the LED for that matter) is based primary on an average temperature of the semiconductor in the LED or “junction temperature” when the LED is emitting light. For example, when the junction temperature is 85° C., LED life is 100,000. That lifetime is halved when junction temperature is increased to 105° C. and doubled when junction temperature is decreased to 55° C. The small size of the LED and its surrounding optics, however, makes measuring the diode junction temperature difficult, costly, and/or impractical to perform with direct methods, such as thermocouples and infrared cameras.
More commonplace, a simple temperature sensor, such as a thermistor, temperature monitoring IC, and thermocouple, is used to read a temperature outside of an LED, like the temperature of a printed circuit board (PCB) on to which the LED is fixed (i.e., the “LED board temperature”). A typical temperature sensor application includes monitoring an LED fixture for an over-temperature situation. For example, a fixture is located outside of a building in Las Vegas and is exposed to a high ambient (external) temperature. In this example application, the temperature sensor is used to trigger some predefined over-temperature condition and shuts the fixture off until some normal operating temperature is retained. In some cases, the temperature sensor may throttle back or decrease light output by the fixture.
In another example application, an LED fixture uses the external temperature as a light output barometer. Meaning, the LED fixture varies the current through an LED in order to try to maintain a certain temperature. The barometer approach attempts to take ambient (external) temperature out of the mix as a variable. For example, the fixture tries to maintain an 85° C. junction temperature maximum. When the ambient (external) temperature is 25° C., the 85° C. junction temperature correlates to the fixture operating at full (100%) output. When the fixture is operating at a higher ambient (external) temperature of 50° C., the fixture may regulate light output (and power) from 100% to 70% in order to maintain the 85° C. junction temperature.
Many LED manufacturers list an estimated LED lifetime on their specification sheets as the amount of time their fixture can run at full (100%) output before an LED lamp inside the fixture is reduced to 70% of the rated light output. These estimations are extremely conservative. Most LED fixtures are routinely controlled and dimmed in some manner (sometimes permanently or long periods of times), and run at less than full (i.e., >100%) output. When an LED fixture is dimmed, its power supply outputs less current to the LED lamp and the lamp appear less bright. Driving the LED with less current lowers the junction temperature experienced by the LED and thus, extends the life of the LED. Simply put, a LED fixture that is dimmed on a regular basis has a far greater lifetime then an LED fixture that is always at full (100%) output.
Simply timing how long time an LED fixture is in use with a timer assumes that an LED is operating at 100% output. In actual use, however, at any given moment, the LED may be outputting less than or more than 100%. In some cases, when the timer reaches the estimated LED lifetime, there is still usable life left in the LED. This may result in waste because it is common to replace the LED when the estimated LED lifetime is reached (e.g. as a part of maintenance schedule or routine). In other cases, the LED is outputting a less-than-acceptable level of light before the timer reaches the estimated LED lifetime. This may result in some LED applications, such as imaging, to perform sub-optimally, or worse, not at all.
In still other cases, the foregoing problems are exacerbated by certain LED fixture designs in which the timer is part of a microprocessor. Even if the fixture is set to “off,” so as long as the microprocessor is running, the microprocessor/timer combination is still clocks lifetime. This works in the “negative” direction because the fixture may be “on” and the microprocessor running for the almost 24 hours, but the fixture may be emitting light for only 8-10 hours of that day.
Clearly, in some cases, simply measuring usage time (e.g., using a timer or timer in a microprocessor) leads to unpredictable results that make commissioning, maintaining, and/or sustaining LED fixtures, in particular, LED lamps, difficult and expensive. Therefore, there is a need for technique for determining and, in some cases, reporting the lifetime of components of an LED lighting fixture that accurately reflects actual use of the components.